In the sealing of flexible film to form packages, such as bags, sealing machines are used which machines are typically hydraulically actuated presses. Heated elements such as a knife edge or a bar having a flat surface are driven into engagement with the film to cut or seal the film. It is not unusual when forming bags from continuous tubular film for a press to have both a knife and a flat bar, to seal and at the same time, cut the film
One type of seal commonly made with flexible side-gussetted packaging is an angle seal. The seal is formed at a 45.degree. or 90.degree. angle to the longitudinal axis of the travelling film. The 45.degree. type of seal allows a bag to be formed with a `square bottom`. The press generally comprises a plate which is received between the opposed facing surfaces of the side gussett(s) (plies). Spaced apart from the outer surfaces of the film are hydraulically actuated plates carrying the sealing bars which bars are in register with one another. In timed sequence the plates close toward one another, the bars contact the film forming an angle seal in each side of the gussett; that is, sealing the two plies on each side.
There are other different types of angle sealers but they all rely upon massive apparatus which requires substantial energy to operate, which energy is independent of the energy needed to heat the sealing bars to seal the film. The prior art is believed to be best represented by U.S. Pat. No. 3,853,664, LaFleur; U.S. Pat. No. 4,055,109, Kan; U.S. Pat. No. 4,261,253, Smith, II; U.S. Pat. No. 4,526,565, Hummel et al; U.S. Pat. No. 4,555,293, French; U.S. Pat. No. 4,649,259, Zurn; U.S. Pat. No. 4,692,135, Johnson; and Netherlands Patent No. 7800-295.
My invention comprises an apparatus and method for forming seals in multi-ply flexible film. The invention uses substantially less energy than prior art sealers and is considerably less expensive than prior art sealers. The sealer embodies a heated wire to effect the seals The current though the wire is preferably maintained at a constant rate. The temperature is such that the film may travel on the wire without the film becoming tacky. When the seal is made substantially all the thermal energy (heat) in the wire is transferred to the film to effect the seal(s). Further, when the seal is made, the film generally envelops the wire; i.e. the contacting plies each surrounding the wire about 180.degree.. After a seal(s) has been effected, the film continues on its travel until the next seal is to be made. The current through the wire remains substantially constant. The timing of the formation of the successive seals and the control of the current through the wire is such that the wire is sufficiently heated to effect the successive seals formed in the film.
The structure of the sealer is such that it may be easily incorporated on existing converting apparatus for single or multiple, side-by-side runs. The hydraulic lines and massive structure of the prior art apparatus are unnecessary.
My invention, in one embodiment, will be described in reference to its use for forming angle seals in continuous tubular film which film is sealed and cut to form block-sealed, side-gussetted, square bottom bags having an aperture on only one side thereof as disclosed in my co-pending application Ser. No. 942,605 filed Dec. 17, 1986 which application is hereby incorporated by reference in its entirety in this application. However, my invention also finds application for sealing multiple films generally as will be described.
My invention, in another embodiment, will be described in reference to its use for forming seals on the handles of tubular, bottom and top welded T-shirt bags, and further my invention includes such bags.
My invention, in still another embodiment, will be described in reference to forming bottom and/or top transverse sides in tubular films and particularly in reference to form two such seals in two separate films simultaneously.
Broadly my invention comprises a hot wire which defines the dimension and shape of the seal to be formed in the plies of a flexible film. A sealing shoe is disposed adjacent the surfaces to be sealed. An electromagnet is disposed on the opposite side of the hot wire. This forms a shoe-wire-electromagnetic array. The plies of the film to be sealed travel between the shoe-wire and between the wire-electromagnet.
The shoe-wire-electromagnet are adapted for relative motion one to the other to effect sealing between the plies of the film. In the prior art the sealing knives, bars, et cetera either disengage the sealing knife from the film after the seal has been made and while the film continues its travel or reduce the current to the sealing bar or both. This is to ensure that the film does not `stick` during its travel. The wire of my invention has a constant current applied to it. The heat transfer characteristics are such that when the seal is made substantially all the heat is transferred to the film being sealed. After the seal has been made the film continues its travel, a portion of the film still contacting the sealing element. When the next seal is made the sealing element has reached the correct temperature to effect the seal in combination with the sealing shoe.
With the apparatus and method of my invention a low cost, low energy sealing is provided at least for the reason that the film may travel on the heated wire at all times and when the seal is formed basically all the heat is transferred from the wire to the film. Thus, the structure and costs associated with disengaging the film from a heated element and/or impulse heating are eliminated and additional energy beyond that necessary to form the seal is not needed.
In the preferred embodiment, a hollow wire is used to form angle seals. The wire is disposed between the plies of a side-gussetted film. The sealing shoe and electromagnet are disposed on the outer sides of the film. The electromagnet is fixed, the wire is biased slightly upward, with reference to horizontal, and the electromagnet is fixed.
The thermal energy required for the seal will vary depending upon the latent heat in the wire, the force exerted by the sealing shoe in its clamping position, the duration of the clamping and the thermoplastic properties of the film.